1. Field of the Invention
The present invention relates to an evaporation cooling module for semiconductor devices wherein plural such semiconductor devices, or chips, mounted on a circuit board are immersed in the coolant liquid so as to be cooled by evaporation of the liquid coolant, one or more heat exchangers immersed in the liquid coolant serving to reliquify the evaporated coolant gas; more particularly, the invention relates to an improved such module having bubble traps for collecting bubbles of evaporated coolant gas and maintaining same in close contact with respectively associated heat exchangers for achieving more effective and efficient reliquification, and bubble guides for guiding gaseous bubbles of the evaporated coolant, produced in passing over corresponding groups of the IC chips, toward the respectively associated bubble traps.
2. State of the Prior Art
The use of liquid cooling modules for cooling plural heated elements is known in the prior art, as illustrated by the following, identified publications. Whereas most of the publications relate to cooling plural packaged devices immersed in the coolant, the publications are applicable as well, in principle, with respect to cooling semiconductor chips which are immersed directly in the coolant.
"Dielectric Bath Promotes Togetherness in IC's" by R. R. Weirather et al., ELECTRONICS, Apr. 17, 1967, describes fundamental principles of cooling plural substrates immersed in a liquid cooling container.
"Better Component Cooling Through Multi Fluid Boiling" by Sevgin Oktay, ELECTRONIC PACKAGING AND PRODUCTION, May, 1970, discloses the fundamental arrangement of cooling a first liquid coolant in which the heated elements are immersed, by a second coolant.
U.S. Pat. No. 3,741,292 to AKALAU ET AL., issued June 26, 1973, discloses a module comprising heat generating components which are exposed within a container, which contains a liquid having a low temperature boiling point.
U.S. Pat. No. 3,851,221 to BEAULIEU ET AL., issued Nov. 26, 1974, discloses a cooling arrangement in which a package of plural, stacked substrates is immersed in a coolant.
Japanese laid-open publications No. 47-37181 to YANATORI, No. 49-98583 to DAIKOKU, and No. 55-91197 to FUJII, in general, disclose a bubble directing guide plate disposed within a liquid coolant for directing gaseous bubbles of the evaporated coolant to travel upwardly along the inner surface of a sealed structure containing the coolant, for increasing the efficiency of cooling.
Techniques for cooling circuit elements of electronic equipment have become increasingly important as the packing density of the elements within the equipment has increased. Various types of cooling methods have been proposed, improvements therein proceeding from air cooling to liquid cooling in view of the greater heat removal afforded by the latter. Early such cooling systems employed a cooling pipe which was coupled to a circuit board or directly to the heat generating elements (e.g., semiconductor chips or other electrical devices which dissipate heat during operation). As the packing density of the elements and the corresponding heat dissipation increased, later arrangements provided for immersing the circuit board itself into a liquid coolant.
In the early stages of the development of liquid immersion cooling techniques, each circuit element was encapsulated in an hermetic case and then mounted on the circuit board, prior to immersion into the liquid coolant. Because of improvements in surface passivation techniques for circuit elements, it has become possible to immerse them directly into the liquid coolant without packaging them in a hermetic case, further increasing the cooling efficiency and the packing density. For example, in computers employing numerous integrated circuits (IC's), it has become possible to mount plural IC chips directly on a ceramic circuit board, without packaging or hermetic sealing of the IC's, and to immerse the latter directly in a liquid coolant. It is also known to bond a group of plural semiconductor chips on a subcircuit board which in turn is bonded to a main, or mother, circuit board, the subcircuit board and its associated plurality of chips being termed, collectively, a chip. Such chips in the form of a subcircuit board and related chips mounted thereon as well are encompassed within the scope of the present invention--i.e., in the following, the words "chip" and "IC chips" encompass either an individual chip or a subcircuit board having a plurality of IC chips mounted thereon.
As is known, liquid immersion cooling may be performed either by evaporation, or evaporative, cooling in which the coolant is caused to boil at the surface of the heat dissipating element, and convection cooling in which the coolant remains in a liquid state and is caused to flow over and around the heat dissipating element. Evaporation cooling is recognized to achieve greater cooling efficiency, although it permits the temperature of the heat dissipating element to be somewhat higher than with convection cooling, other factors remaining equal. However, by choosing a liquid coolant having a sufficiently low boiling point, the heat dissipating element may be maintained at a sufficiently low temperature thereby to assure normal operating conditions. For example, IC chips typically are made to withstand an operating temperature of 80.degree. C.; thus, by using a liquid coolant having a boiling point less than 80.degree. C., evaporation cooling may be employed, permitting the utilization of its greater efficiency while maintaining the requisite operating temperature of the IC chips.
Liquid coolants used for liquid immersion cooling must be noncorrosive and electrically insulating. Suitable such coolants are Freon (C.sub.2 Cl.sub.3 F.sub.3) which has a boiling point of approximately 49.degree. C., and various fluorocarbons such as C.sub.5 F.sub.12 having a boiling point of approximately 30.degree. C., and C.sub.6 F.sub.14 having a boiling point of approximately 56.degree. C.
In practical operation of a cooling chamber, or module, employing evaporation cooling, the evaporated coolant gas must be reliquified and circulated back as a liquid to the cooling chamber. The efficiency with which the reliquification of the coolant gas is achieved therefore is an important factor affecting the overall efficiency of the liquid coolant system. Typically, the evaporated coolant gas is reliquified by a second coolant, such as chilled water, which passes through a heat exchanger over which the evaporated coolant gas flows. FIG. 1 herein illustrates schematically a conventional liquid immersion cooling chamber 1. Case 1 houses a plurality of circuit boards 4 typically of ceramic material, each having mounted thereon plural IC chips 3, which are supported by corresponding connectors 2 and immersed within a liquid coolant 5. The heat dissipating IC chips 3 are cooled by the liquid coolant 5 which in turn is caused to evaporate; the resultant gas collects in the space 7 above the surface of the liquid coolant 5 and therein is caused to be reliquified by interaction with the heat exchanger 6.
The general structural arrangement illustrated in FIG. 1, in which the heat exchanger 6 occupies the space 7 above the surface of the liquid coolant 5 for contacting the evaporated coolant gas, is known to provide improved efficiency as compared with systems in which a heat exchanger is immersed within the liquid coolant. However, various problems are encountered in such systems, which result in gradual reduction of the reliquification efficiency and ultimately loss of the cooling effect and correspondingly an unexpectably short cooling system life.
Among the reasons believed to contribute to the unacceptably short coolant system life is the fact that in such liquid immersion cooling systems, many elements are immersed in the coolant including IC chips, circuit boards, terminal boards forming portions of the cooling chamber walls, and the like. Particularly, various gases, primarily air and water vapor captured as moisture in the various elements, are released from those elements and travel through the coolant 5 into the upper gas space 7 in which the heat exchanger 6 is mounted. Since those released gases are not reliquified by the heat exchanger, they accumulate and surround the heat exchanger 6, reducing its reliquification efficiency. It is also expected that slow leaks may occur in such cooling chambers. Reduction of the reliquification efficiency ultimately may result in an unacceptable increase in the internal operating temperatures within the chamber which, of course, is unacceptable in view of the required maintenance of predetermined operating temperatures of the various devices, e.g., ceramics, semiconductors, insulating materials and so forth, sealed within the chamber. It appears impossible with present state-of-the-art technology either to effect perfect evacuation of the undesired, released gases within the chamber, or to avoid leaks of the chamber so as to insure that only the desired coolant, either in its liquid or gaseous state, is present within the chamber. Therefore, degassing must be performed on a repeated, periodic schedule, for removal of such undesired, released gases in order to maintain high cooling efficiency.
Whereas the problem of reduced reliquification efficiency is avoided by systems in which the heat exchanger is immersed in the liquid coolant, the coolant gas must be reliquified by the coolant liquid itself with the result that the reliquification efficiency again is reduced. Moreover, since the heat dissipating elements are cooled merely by contact with the convection flow of liquid coolant thereover, the liquid coolant must be maintained at a lower temperature than its boiling point since the benefits of evaporation cooling are lost.